Mass Flow Controllers (MFC) are used extensively in modern semiconductor manufacturing to control the flow of various gases into a wide variety of process equipment, such as etchers, deposition reactors, implanters, etc. Many individual pieces of equipment have more than one such MFC, with six to eight MFCs per major piece of semiconductor manufacturing equipment not being uncommon.
A representative MFC 100 is shown in three orthogonal views in FIGS. 1A, 1B, and 1C. A source of gas is connected to and received at an input port 102 and a controlled flow is delivered to an output port 104 and to downstream equipment attached thereto. A housing 106 contains an electronic module for controlling the MFC which is connected to a system controller (not shown) by way of a cable attached to an MFC card edge connector 108.
To more fully appreciate the invention, a brief description of the inner workings of a typical MFC is warranted. Referring now to FIG. 2, the representative MFC 100 includes four distinct subsystems. A mass flow sensor 120 produces an electrical signal proportional to the mass of the gas flowing therethrough. A control valve 122 acts as a variable orifice to control the total gas flowing through the MFC 100. A flow bypass 124 diverts a small portion of the total gas flowing into the MFC 100 into and through the mass flow sensor 120. Lastly, a printed wiring board 126 contains various circuits for controlling the MFC 100, and typically include a sensor bridge and amplifier circuit, a control circuit, and an RC network, as described more fully below.
Referring now to FIG. 3, the basic operation of the MFC 100 is described. The mass flow sensor 120 is illustrated as a "capillary tube" thermal mass flow sensor which is designed to measure the mass of gas flowing through a thin stainless steel capillary sensor tube 131. Two temperature sensing wire windings 130, 132 are attached around the sensor tube 131. One winding is located on the upstream side of the sensor tube 131 and the other winding is located on the downstream side of the sensor tube 131. The wire forming these windings 130, 132 is resistance thermal detection (RTD) type wire, which means the resistance of such wire is a function of the temperature of the wire. The sensor tube is installed in a protective cover and is usually enclosed in heat insulating material. An equal amount of heat is produced in both sensor windings either directly by a constant current source or by using a separate heater wire winding (not shown) between the upstream sensor winding 130 and the downstream sensor winding 132.
With no gas flowing through the sensor 120, both the upstream sensor winding 130 and the downstream sensor winding 132 are at the same temperature, and consequently, they both have the same resistance. Since each winding 130, 132 has the same resistance, and the same current flows through each winding, the voltage drop across each winding 130, 132 is the same. The voltage drop across each winding 130, 132 is compared by a sensor bridge and amplifier circuit 134 to produce an MFC output voltage conveyed on sensor output terminal 140.
With 50% gas flow through the mass flow sensor 120, gas at room temperature flows through the sensor tube 131 and heat from the upstream sensor winding 130 is transferred to the gas molecules. This reduces the temperature of the upstream sensor winding 130, and increases the temperature of the gas. As this hotter gas flows past the downstream sensor winding 132 it transfers less heat away from the downstream sensor winding 132. This difference in temperature between the upstream sensor winding 130 and the downstream sensor winding 132 results in a difference in resistance between the two windings 130, 132, which then results in a difference in voltage across the two windings 130, 132. This voltage difference is amplified and linearized by sensor bridge and amplifier circuit 134 to become the MFC output voltage at sensor output terminal 140. This output voltage is an indirect result of gas molecules flowing through the mass flow sensor 120.
In other words, the difference in temperature between the upstream sensor winding 130 and the downstream sensor winding 132 is sensed as a small (millivolts) non-zero voltage by the sensor bridge and amplifier circuit 134. This small voltage is amplified to a typical level of several volts and linearized to provide a 0 to 5 volt DC output voltage signal (for many commercial MFCs) which is proportional to the mass of the gas flowing through the mass flow sensor 120. If the ratio of gas flowing through the mass flow sensor 120 and through the flow bypass 124 is correct, the output signal is proportional to the mass of the gas flowing through the MFC 100 from the input port 102 to the output port 104.
A control circuit 136 compares the output voltage signal produced by the sensor bridge and amplifier circuit 134 against an externally supplied setpoint signal conveyed on terminal 142. The setpoint signal is usually a 0 to 5 volt DC signal and corresponds to the actual flow desired through the MFC. The control circuit 136 drives a valve control transistor 138 which positions the control valve 122 in such a manner as to eliminate any difference between the setpoint signal and the output signal. If the actual flow (represented by the MFC output voltage) is less than the desired flow (as represented by the MFC setpoint voltage), the control circuit 136 biases the valve control transistor 138 in such a manner as to open the control valve 122 to allow more gas flow through the control valve 122, and hence through the MFC 100. As more gas flows through the MFC 100, proportionally more gas flows through the mass flow sensor 120 causing the MFC output voltage to increase. This reduces the difference between the MFC setpoint voltage and the MFC output voltage.
The electrical schematic diagram of a particular manufacturer's MFC is shown in FIG. 4. Shown is the schematic for a model FC-2950M mass flow controller/flowmeter available from Tylan General, Inc., located in San Diego and Torrance, Calif. The upstream sensor winding 130 and downstream sensor winding 132 are shown as resistors which connect into a bridge circuit 154, the outputs of which are then amplified by an amplifier 156 to produce the MFC output voltage at terminal 140. Various RC feedback circuitry within the amplifier 156 serve to stabilize the operation of the amplifier 156. The bridge circuit 154, amplifier 156, and other feedback and reference circuits shown form the sensor bridge and amplifier circuit 134 described previously. The control circuit 136 receives the MFC output voltage and compares it to the MFC setpoint voltage on terminal 142 to control the valve control transistor 138, whose output terminal 150 is connected to the control valve 122 (modeled on the schematic as a resistor connected to terminals V1 and V2) through a current-limiting resistor 158. Each of the signals shown on the left side of FIG. 4 are usually available at the MFC card edge connector 108.
As described above, the closed-loop feedback operation of the control circuit 136 causes the MFC output voltage on terminal 140 to be driven to match the MFC setpoint voltage on terminal 142. The "valve voltage" on terminal 150 is adjusted to whatever voltage is required to adjust the gas flow in order to cause the MFC output voltage to match the MFC setpoint voltage. A system controller which is connected to the MFC card edge connector 108 presents to the MFC 100 the desired MFC setpoint voltage, and then monitors the MFC output voltage produced by the MFC 100 in response thereto.